This invention relates to porous, resin bonded abrasive tools suitable for surface grinding and polishing of hard materials, such as ceramics, metals and composites comprising ceramics or metals. The abrasive tools are useful in backgrinding of silicon and alumina titanium carbide (AlTiC) wafers used in the manufacture of electronic components. These abrasive tools grind ceramics and semi-conductors at commercially acceptable material removal rates and wheel wear rates with less workpiece damage than conventional superabrasive tools.
An abrasive tool designed to yield faster and cooler cutting action during grinding is disclosed in U.S. Pat. No. 2,806,772. The tool contains about 25 to 54 volume percent abrasive grain in about 15 to 45 volume percent resin bond. The tool also contains about 1-30 volume percent of pore support granules, such as vitrified clay thin walled hollow spheres (e.g., Kanamite balloons) or heat expanded (intumescent) perlite (volcanic silica glass) to separate the abrasive grain particles for better cutting and less loading of the grinding face with debris from the workpiece. The pore support granules are selected to be about 0.25 to 4 times the size of the abrasive grain.
An abrasive tool containing only fused alumina bubbles and no abrasive grain is disclosed in U.S. Pat. No. 2,986,455. The tool has an open, porous structure and free-cutting characteristics. Resin bonded wheels made according to the patent are used to grind rubber, paper fiber board and plastics.
Erodable agglomerates useful in making abrasive tools are disclosed in U.S. Pat. No. 4,799,939. These materials contain abrasive grain in resin bond materials and up to 8 weight percent hollow bubble material. The agglomerates are described as being particularly useful in coated abrasives.
An abrasive tool suitable for grinding surfaces of sapphire and other ceramic materials is disclosed in U.S. Pat. No. 5,607,489 to Li. The tool is contains metal clad diamond bonded in a vitrified matrix comprising 2 to 20 volume % of solid lubricant and at least 10 volume % porosity.
The abrasive tools known in the art have not proven entirely satisfactory in fine precision surface grinding or polishing of ceramic components. These tools fail to meet rigorous specifications for part shape, size and surface quality in commercial grinding and polishing processes. Most commercial abrasive tools recommended for use in such operations are resin bonded superabrasive wheels designed to operate at relatively low grinding efficiencies so as to avoid surface and subsurface damage to the ceramic components. These commercial tools typically contain over 15 volume percent diamond abrasive grain having a maximum grain size of about 8 microns. Grinding efficiencies are further reduced due to the tendency of ceramic workpieces to clog the wheel face, requiring frequent wheel dressing and truing to maintain precision forms.
As market demand has grown for precision ceramic and semi-conductor components in products such as electronic devices (e.g., wafers, magnetic heads and display windows), the need has grown for improved abrasive tools for fine precision grinding and polishing of ceramics and other hard, brittle materials.
The invention is an abrasive tool comprising a backing and an abrasive rim containing a maximum of about 2 to 15 volume percent abrasive grain, the abrasive grain having a maximum grit size of 60 microns, wherein the abrasive rim comprises resin bond and at least 40 volume percent hollow filler materials, and the abrasive grain and resin bond are present in the abrasive rim in a grain to bond ratio of 1.5:1.0 to 0.3:1.0.
The abrasive tools of the invention are grinding wheels comprising a backing having a central bore for mounting the wheel on a grinding machine, the backing being designed to support a resin bonded abrasive rim along a peripheral grinding face of the wheel. The backing may be a core disc or ring formed into a planar shape or into a cup shape, or an elongated spindle or some other rigid, preformed shape of the type used to make abrasive tools. The backing is preferably constructed of a metal, such as aluminum or steel, but may be constructed of polymeric, ceramic or other materials, and may be a composite or laminate or combination of these materials. The backing may contain particles or fibers to reinforce the matrix, or hollow filler materials such as glass, silica, mullite, alumina and Zeolite(copyright) spheres to reduce the density of the backing and reduce the weight of the tool.
Preferred tools are surface grinding wheels, such as type 2A2T superabrasive wheels. These tools have a continuous or a segmented abrasive rim mounted along the narrow lip of a ring- or cup-shaped backing. Other abrasive tools useful herein include type 1A superabrasive wheels having a planar core backing with an abrasive rim around the outer circumference of the core, inner diameter (I.D.) grinding abrasive tools with an abrasive rim mounted on a shank backing, outer diameter (O.D.) cylindrical grind finishing wheels, surface grinding tools with abrasive xe2x80x9cbuttonsxe2x80x9d mounted on a face of a backing plate, and other tool configurations used to carry out fine grinding and polishing operations on hard materials.
The backing is attached to the abrasive rim in a variety of ways. Any cement known in the art for attaching abrasive components to metal cores, or to other types of backings, may be used. A suitable adhesive cement, Araldite(trademark) 2014 Epoxy adhesive is available from Ciba Specialty Chemicals Corporation, East Lansing, Mich. Other means of attachment include mechanical attachment (e.g., abrasive rim may be mechanically screwed to the backing plate through holes placed around the rim and in the backing plate, or by dovetail construction). Slots may be grooved into the backing element and the abrasive rim, or abrasive rim segments, if the rim is not continuous, may be inserted into the slots and fastened in place by an adhesive. If the abrasive rim is used in the form of discrete buttons for surface grinding, the buttons also may be mounted onto the backing with an adhesive or by mechanical means.
The abrasive grain used in the abrasive rim is preferably a superabrasive selected from diamond, natural and synthetic, CBN, and combinations of these abrasives. Also useful herein are conventional abrasive grains, including, but not limited to alumina oxide, sintered sol gel alpha alumina, silicon carbide, mullite, silicon dioxide, alumina zirconia, cerium oxide, combinations thereof, and mixtures thereof with superabrasive grains. Finer grit abrasive grains, i.e., a maximum grain size of about 120 microns, are useful. A maximum size of about 60 microns is preferred.
Diamond abrasives are used to grind ceramic wafers. Resin bond diamond types are preferred (e.g., Amplex diamond available from Saint-Gobain Industrial Ceramics, Bloomfield, Conn.; CDAM or CDA diamond abrasive available from DeBeers Industrial Diamond Division, Berkshire, England; and IRV diamond abrasive available from Tomei Diamond Co., Ltd., Tokyo, Japan).
Metal coated (e.g., nickel, copper or titanium) diamond can be used (e.g., IRM-NP or IRM-CPS diamond abrasive available from Tomei Diamond Co., Ltd., Tokyo, Japan; and CDA55N diamond abrasive available from DeBeers Industrial Diamond Division, Berkshire, England).
Grain size and type selection will vary depending upon the nature of the workpiece, the type of grinding process and the final application for the workpiece (i. e., the relative importance of material removal rate, surface finish, surface flatness and subsurface damage specifications dictate grinding process parameters). For example, in the backgrinding and polishing of silicon or AlTiC wafers, a superabrasive grain size ranging from 0/1 to 60 micrometers (i.e., smaller than 400 grit on Norton Company diamond grit scale) is suitable, 0/1 to 20/40 microns is preferred, and 3/6 microns is most preferred. Metal bond, or xe2x80x9cblockyxe2x80x9d, diamond abrasive types may be used (e.g., MDA diamond abrasive available from DeBeers Industrial Diamond Division, Berkshire, England). Finer grit sizes are preferred for surface finishing and polishing the back face of a ceramic or semiconductor wafer after electronic components have been attached to the front face of the wafer. In this range of diamond grain sizes, the abrasive tools remove material from silicon wafers and polish the surface of the wafer, but the abrasive tools do not remove as much material from AlTiC wafers due to the hardness of AlTiC wafers. The tools of the invention have achieved a surface finish polish as smooth as 14 angstroms on AlTiC wafers.
In the tools of the invention, the hollow filler material is preferably in the form of friable hollow spheres such as silica spheres or microspheres. Other hollow filler materials useful herein include glass spheres, bubble alumina, mullite spheres, and mixtures thereof. For applications such as backgrinding silicon wafers, silica spheres are preferred and the spheres are preferably larger in diameter than the size of the abrasive grain. In other applications, hollow filler materials may be used in diameter sizes larger than, equivalent to or smaller than the diameter size of the abrasive grain. A uniform diameter size may be obtained by screening commercially available fillers, or a mixture of sizes may be used. Preferred hollow filler materials for silicon wafer grinding may range from 4 to 130 micrometers in diameter. Suitable materials are available from Emerson and Cuming Composite Materials, Inc., Canton MA (Eccosphere(trademark) SID-311Z-S2 silica spheres, 44xcexc average diameter spheres).
The abrasive grain and hollow filler material are bonded together with a resin bond. Various powdered filler materials known in the art may be added to the resin bond materials in minor amounts to aid in manufacturing the tools or to improve grinding operations. The preferred resins for use in these tools include phenolic resins, alkyd resins, polyimide resins, epoxy resins, cyanate ester resins and mixtures thereof. Suitable resins include Durez(trademark) 33-344 phenolic powdered resin available from Occidental Chemical Corp., North Tonawanda, N.Y.; Varcum(trademark) 29345 short flow phenolic resin powder available from Occidental Chemical Corp., North Tonawanda, N.Y.
Preferred resins for tools containing a high volume percentage of hollow filler materials (e.g., 55 to 70 volume percent spheres) are those having the ability to wet the surface of the silica and abrasive and readily spread over the surface of the silica spheres so as to adhere diamond abrasive to the surface of the spheres. This characteristic is particularly important in wheels comprising very low volume percentages of resins, such as 5-10 volume percent.
As a volume percentage of the abrasive rim, the tools comprise 2 to 15 volume % abrasive grain, preferably 4 to 11 volume %. The tools comprise 5 to 20 volume % resin bond, preferably 6 to 10 volume %, and 40 to 75 volume % hollow filler material, preferably 50 to 65 volume %, with the balance of the resin bond matrix comprising residual porosity following molding and curing (i.e., 12 to 30 volume % porosity). The ratio of diamond grain to resin bond may range from 1.5:1.0 to 0.3:1.0, and preferably is from 1.2:1.0 to 0.6:1.0.
The abrasive rim of the tools of the invention are manufactured by uniformly mixing the abrasive grain, hollow filler material and resin bond, and molding and curing the mixture. The abrasive rims may be manufactured by dry blending the components, with the optional addition of wetting agents, such as liquid resole resins, with or without a solvent, such as water or benzaldehyde, to form an abrasive mixture, hot pressing the mixture in a selected mold and heating the molded abrasive rim to cure the resin and create an abrasive rim effective for abrasive grinding. The mix is typically screened before molding. The mold is preferably constructed of stainless steel or high carbon- or high chrome-steel. For wheels having 50-75 volume % hollow filler material, care must be exercised during molding and curing to avoid crushing the hollow filler materials.
The abrasive rim preferably is heated to a maximum temperature of about 150 to 190xc2x0 C. for a period of time sufficient to crosslink and cure the resin bond. Other similar curing cycles also may be employed. The cured tool is then stripped from the mold and air-cooled. The abrasive rim (or buttons or segments) are attached to a backing to assemble the final abrasive tool. Finishing or edging steps and truing operations to achieve balance may be carried out on the finished tool.
By means of resin and filler selections and curing conditions, the resin bond may be rendered relatively brittle or friable, and will break or chip faster and the abrasive tool will have less of a tendency to load with grinding debris. Commercial abrasive tools for finishing ceramic or semi-conductor wafers often need to be dressed with dressing tools to clear accumulated grinding debris from the grinding face. In microabrasive grain wheels, such as the wheels of the invention, the dressing operation often wears away the wheel faster than the grinding operation. Because dressing operations are needed less frequently with the resin bonded tools of the invention, the tools are consumed more slowly and have a longer life than resin bonded tools used in the past, including wheels having higher diamond content or a stronger, less friable bond. The most preferred tools of the invention have cured bond properties that yield an optimum balance of tool life with brittleness or tendency of the bond to fracture during grinding.
Tools made with higher volume percentages of hollow filler material (e.g., 55 to 70 volume percent) are self-dressing during surface grinding and polishing operations on ceramic or semi-conductor wafers. It is believed that the incoming rough ceramic or semi-conductor wafer acts in the manner of a dressing tool to open the face of the grinding tool and release debris loaded on the face. Thus, in typical commercial operations, each new workpiece initially presents a rough surface to dress the tool and then as grinding progresses, debris begins to load the face and the tool begins to polish the workpiece surface and the power consumption begins to increase. With the tools of the invention, this cycle occurs within the power tolerances of the grinding machines and without causing workpiece burn. At the completion of the cycle with one workpiece, a new, rough surface on the next workpiece is presented to dress the face of the tool and the cycle is repeated. This capacity of the tools of the invention to grind the surface of ceramic or semi-conductor wafers without a dressing operation offers a significant benefit in the manufacture of ceramic or semi-conductor wafers.
With lower contents of hollow filler material (i. e., less than 55 volume percent), the tools of the invention require a dressing operation as the ceramic wafers are ground to a finer surface finish, because the wafer tends to load the face of the abrasive tool and power consumption increases.
The tools of the invention are preferred for grinding ceramic materials including, but not limited to, oxides, carbides, silicides such as silicon nitride, silicon oxynitride, stabilized zirconia, aluminum oxide (e.g., sapphire), boron carbide, boron nitride, titanium diboride, and aluminum nitride, and composites of these ceramics, as well as certain metal matrix composites such as cemented carbides, polycrystalline diamond and polycrystalline cubic boron nitride. Either single crystal ceramics or polycrystalline ceramics can be ground with these improved abrasive tools.
Among the ceramic and semi-conductor parts improved by using the abrasive tools of the invention are electronic components, including, but not limited to, silicon wafers, magnetic heads, and substrates.
The tools of the invention may be used for polishing or finish grinding of components made from metals or other hard materials.